1. AP Environmental Science Community Interactions, Species Diversity and Succession

2. Community Ecology
Community = an association of interacting species inhabiting some defined area
Community structure = includes attributes such as the number of species and the kinds of species comprising a community
Community function = includes attributes such as energy flow through food webs
Are ecological communities “real” functional units?

3. Yes! Frederick E. Clements (1874 – 1945)
Holistic community concept (Closed community)
Community as a superorganism
Based his conclusion on a study of vegetation zones along altitudinal gradients
Basis of the modern-day “Gaia concept”

4. Vegetation Zones

5. Closed Communities
Along an environmental gradient (e.g. temperature, moisture) there are sometimes sharp boundaries between species assemblages
Areas between assemblages are known as ecotones
Best seen where there are sharp physical boundaries between species assemblages
Between aquatic and terrestrial areas
Between distinct soil types
Between north and south facing slopes

6. Closed Communities

7. No! Henry A. Gleason ( )
Individualistic concept (Open community)
The appearance of communities are simply due to the individual interactions of species and don’t reflect any innate organization above the species level

8. Open Communities
Closer examination of the deciduous forests of eastern North America revealed no sharp boundaries between forest types
There were generally two gradients of forest types:
A north-south gradient based on temperature
An east-west gradient based on moisture
Gradient analysis is a technique invented by Robert H. Whittaker to analyze changes in (plant) species abundance along gradients
Gradients are variables that affect plant community structure (i.e temperature, light & water availability, soil nutrients, etc.)
Put to rest the extreme closed community concept of Clements

9. Open Communities

10. Factors Affecting Biological Community Structure
Species diversity – function of the number of different species that a community contains (species richness) and the relative abundance of individuals of each species (species evenness).
Niche structure:
J.H. Brown (1981) bases this on ‘capacity rules’ (how many potential ecological niches occur) and ‘allocation rules’ (how these ecological niches can be divided up among species).
Geographical location
Consistent climates foster high diversity but ‘specialist species’
Variable climates foster low diversity but ‘generalist species’

11. Species Richness
Species Richness is a measure of the total number of different species in a community
Observations regarding species richness:
Gradual increase in diversity/richness Pole to Equator
In a given ecosystem, species richness increases over time.
Larger ecosystems tend to have greater species richness.
Usually increases with ecological succession.
Why these trends?

12. Why These Trends? Productivity? Spatial complexity/heterogeneity?
However, an aspect of environmental structure that is important to one group may not be important to another group
Therefore, you must know something about the ecological requirements of species to predict how environmental structure affects their diversity.

13. Species Richness Factors that affect species richness saturation
rates of colonization and extinction (MacArthur & Wilson (1967)
Depend on size of ecosystem and
Distance from a source of colonists
disturbance
severity, variability, and predictability (Slobodkin and Sanders, 1969)
i.e. rainforests – favorable and constant; therefore predictable; polluted stream – severe and variable; therefore unpredictable

14. Calculating Species Richness
Generally exclude “accidental species” in the count
Does not take into account abundance patterns among the various species
Species counts depend on the sample size or sample area (rarely do you get every one!)
“Equilibrium model of island biogeography” (MacArthur & Wilson, 1967)
# of species (species richness) on an “island” is determined by immigration and emigration of species
Larger islands and islands closer to “mainland” will have greater species richness.

15. Equilibrium Model of Island Biogeography

17. Species-Area Curves Species-area curve S = CAz log S = log C + z log A
An equation for a straight line where log c = Y intercept and z = the slope of the line S
Species Richness C
constant A
Area z

18. Species Area Curves

19. Diversity Indices Shannon-Weiner Index
Also known as the “Shannon Index” or the “Shannon-Weaver Index”
Based on Information Theory
Measures the amount of order in a system in bits of information
Determines rarity or commonness of species
Combines measure or species richness with species evenness.

20. Calculating the Shannon-Weiner Index
H = -∑(pi) (ln pi)
H = Shannon-Weiner Diversity Index
pi = proportion of total sample belong to the ith species
ln pi = the natural log of pi

21. An Example
Community A: 99 individuals in species 1; 1 individual in species 2
“Uneven” community
H = -[(p1)(ln p1) + (p2)(ln p2)]
H = -[0.99 (ln 0.99) (ln 0.01)]
H = -[( ) + ( )] =
Community B: 50 individuals in species 1; 50 individuals in species 2
“Even” community
H = -[(0.5) (ln 0.5) + (0.5) (ln 0.5)]
H = -[( ) + ( )] =

23. Pielou’s Evenness Index
Compares the evenness component of species diversity in a community with the maximum amount of evenness possible given the same species richness
Also known as Shannon’s equitability (EH)
EH = H/Hmax = H/ln S
Hmax = ln S
Community A--> J = 0.056/0.693 = 0.081
Community B--> J = 0.693/0.693 = 1.000
EH ranges from 0-->1.0 with 1.0 indicating maximum evenness

25. Use of Diversity Indices
Used in applied ecology to measure differences in diversity between two or more communities
Also used to measure changes in diversity within a community after some environmental modification or disturbance

26. Robert MacArthur (1930-1972) UPenn & Princeton
Studied the diversity of warblers in northeastern forests
Discovered that different warbler species fed in different parts of the forest canopy

27. MacArthur’s Warblers

28. Bird Species Diversity vs. Foliage Height Diversity
Also showed that the greater the Foliage Height Diversity of the forest the greater the Bird Species Diversity

29. Ecological Succession
Plant (and animal) communities develop in often predictable ways following disturbances, a process known as succession
Natural, gradual changes in the types of species that live in an area
Classified as primary or secondary succession

30. Indiana Dunes Succcession
Henry Chandler Cowles ( )
American botanist and ecological pioneer
Born in Kensington, Connecticut, he attended Oberlin College in Ohio and the University of Chicago
He obtained his Ph.D. in 1898 for his study of vegetation succession on the Lake Michigan sand dunes.

31. Kinds & qualitative characteristics of disturbances that impact communities
Asteroid impact
Volcanism
Disturbance magnitude
Glaciation
Landslide
Hurricane
Fire
Ice Storm
Treefall
Disturbance frequency

32. Primary Succession General characteristics
Begins in a place without any soil
Sides of volcanoes
Landslides
Flooding
Remnants of glaciers
Starts with the arrival of living things such as lichens that do not need soil to survive
Lichens (pioneer species) secrete weak acids which in turn break rocks into soil
Called the Pioneer Stage

33. Lichens: Symbiosis of a fungus & an alga
Example of a mutualism

34. Primary Succession General characteristics
Soil starts to form as lichens and the forces of weather and erosion help break down rocks into smaller pieces
When lichens die, they decompose, adding small amounts of organic matter to the rock to make soil

35. Primary Succession General characteristics
Simple plants like mosses and ferns can grow in the new soil

36. Primary Succession General characteristics
The simple plants die, adding more organic material
The soil layer thickens, and grasses, wildflowers, and other plants begin to take over
Called Seral Stages

37. Primary Succession General characteristics
These plants die, and they add more nutrients to the soil
Shrubs and trees can survive now
Still called Seral Stages

38. Primary Succession General characteristics
Insects, small birds, and mammals have begun to move in
What was once bare rock now supports a variety of life
Called a Climax Community

39. Climax Community
A stable group of plants and animals that is the end result of the succession process
Does not always mean big trees
Grasses in prairies
Cacti in deserts
Mosses, sedges and lichens in the tundra

40. Secondary Succession
Begins in a place that already has soil and was once the home of living organisms
Occurs faster and has different pioneer species than primary succession
Example: after forest fires

42. Intermediate Disturbance Hypothesis
J.P. Grime (1973), Henry Horn (1975) and Joseph Connell (1978)
The Intermediate Disturbance Hypothesis is an ecological hypothesis which proposes that biodiversity is highest when disturbance is neither too rare nor too frequent.
With low disturbance, competitive exclusion by the dominant species arises.
With high disturbance, only species tolerant of the stress can persist.

43. Role of Disturbance in Succession